Concrete Member - Design

Concrete design and optimization can be performed for standard concrete shapes based on the following codes:

• The 2022, 2019, 2014, 2011, 2008, 2005, 2002 and 1999 Editions of ACI 318
• The 1997 Edition of the British code (BS 8110)
• The 1992 EuroCode (EC2) and the British publication of the 2014 and 2004 Eurocode (BSEN)
• The 1994 and 2004 Editions of the Canadian code (CSA A23.3)
• The 2000 Edition of the Indian code (IS 456)
• The 2001 Edition of the Australian code (AS 3600)
• The 1995 Edition of the New Zealand code (NZS 3101)
• The 2004 Edition of the Mexican code (NTC-DF)
• The 2007 Edition of the Saudi Building Code (SBC 304)

The program designs the longitudinal and shear reinforcement for rectangular beams and rectangular or circular columns. These calculations encompass all the code requirements except those noted in the Limitations section of this document. The program also provides reinforcement detailing information for concrete beams and interaction diagrams for concrete columns in the member detail reports.

Concrete Parameters (General)

• Unbraced Lengths
• K Factors (Effective Length Factors)

ACI 318 Design Parameters

• Cm Coefficients (Interactive Bending Coefficients)
• Sway Flags
• Icr Factors (Cracked Moment of Inertia Factors)

British Eurocode Design Parameters

Known Limitations

• ACI
• Canadian
• British
• Indian
• EuroCode
• Australian / New Zealand

Apply a Concrete Design Code

To Apply a Concrete Design Code

1. On the Codes tab of ‘Model Settings’ window.
2. Choose the Concrete code from the drop down list.
3. Click Apply or OK.

Concrete Spans

Beam member types are supported by the following: Vertical Boundary Conditions (Fixed, Reaction), Column Members, Near Vertical Plate Elements, and other Beam Members that are supporting that member. 

Column member types are supported by the following: Horizontal Boundary Conditions (Fixed, Reaction, Spring), Beam Members, Near Horizontal Plate Elements, and Rigid Diaphragms.

Concrete Design Parameters - Columns 

The Concrete Column tab on the Members Spreadsheet records the design parameters for the code checks of concrete columns. These parameters can also be assigned graphically. See Modifying Member Design Parameters to learn how to do this.

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The following parameters can be defined for each concrete column.

Label

You can assign a unique Label to all of the members. Each label must be unique, so if you try to enter the same label more than once you get an error message. You can relabel at any time with the Relabel options on the Tools menu.

Shape

The member Shape or Section Set is reported in the second column.  This value is listed for reference only and can not be edited as it is dictated by the entry in the Section/Shape column on the Primary tab.

Length

The member Length is reported in the third column.  This value can not be edited as it is dependent on the member end coordinates listed on the Primary Data tab. It is listed here as a reference for unbraced lengths which are discussed in the next section.

Unbraced Length

You can specify unbraced lengths or have RISA-3D calculate them for you. The unbraced lengths are Lu-yy and Lu-zz.

The Lu values, Lu-yy and Lu-zz, represent the unbraced length of column members with respect to column type buckling about the member's local y and z axes, respectively. These Lu values are used to check the column for Euler buckling, and for the Moment Magnification Procedure in older editions of the ACI code.

If left blank, these unbraced lengths all default to the member's full length.

For physical members, you can enter the code “Segment” in the unbraced length fields and the length of each segment is used. A “segment” is the distance between the joints that are on the physical member. For example, suppose you have a physical member that is 20 feet in length, and there are two joints along the physical member, one 5 feet from the end and one at 15 feet. An unbraced length of 5 feet is used for the first segment, then a value of 10 feet is used in the middle segment, and again a value of 5 feet would be used in the last segment.

For additional advice on this topic, please see the RISA Tips & Tricks website: www.risa.com/post/support. Type in Search keywords: Unbraced Lengths.

K Factors (Effective Length Factors)

The K Factors are also referred to as effective length factors. Kyy is for column type buckling about the member's local y-y axis and Kzz is for buckling about the local z-z axis. 

If a value is entered for a K Factor, that value is used for the entire length of the physical member. If an entry is not made (left blank), the value internally defaults to '1' for that member. See ACI 318-14 Section R6.2.5 (ACI 318-11 Section R10.10.1) for an explanation of how to calculate K Factors.

RISA-3D is able to approximate the K-values for a column based on the member's sway condition and end release configuration. The K-factor approximation is based on the idealized tables given in the AISC steel specification. The following table gives the values used for various conditions.

RISA-3D recognizes a pinned boundary condition for the K approximation for a full pin, i.e. if all the rotations in the boundary condition are released.  If any of the rotations in a boundary condition are restrained, the boundary condition is considered “fixed” for the K approximation.

Any configuration not described here is given the default value of 1.0.

If any value that influences these K values is changed, the K approximation should be redone. For instance, if you have RISA-3D approximate K-values then change some end release designations, you should redo the K approximations.

Remember that the K-values are approximations, and you should check to make sure you agree with all K-values RISA-3D assigns.  You can always override a K-value after an approximation by directly entering the value that you want in the appropriate field.  Keep in mind that a subsequent approximation will overwrite any manually input values so you need to override the approximation each time it is performed.

Limitation:

RISA-3D currently neglects the influence of adjoining framing members when those members are connected at a joint that also has degrees of freedom restrained by boundary conditions.  For example, suppose a column and beam member connect at a joint that is restrained for translation in all directions (I.e. the joint is “pinned”).  The K factor approximation neglects the beam member when it calculates the K factor for the column and vice versa. The effect is, the ends of the members at that joint are seen as “pinned” and not “fixed” for the K-factor approximation.

Sway Flags

The Sway Flags indicate whether the member is to be considered subject to sidesway for bending about its local y and z axes. The y sway field is for y-y axis bending and the z sway field is for z-z axis bending. Click on the field to check the box and indicate that the member is subject to sway for that particular direction, or leave the entry blank if the member is braced against sway. These sway flags influence the calculation of the K Factors as well as the Cm.

Cm – Equivalent Moment Correction Factor

The Cm Coefficients are used to check the column for Euler buckling, and for the Moment Magnification Procedure in older editions of the ACI code.  Cm-yy is for bending about the columns's local y-y axis and Cm-zz is for bending about the local z-z axis. If these entries are left blank they are automatically calculated.

In the ACI design code, the Cm values are only applicable for non-sway frames. Therefore, this value is ignored if the corresponding sway flag is checked.

Flexural and Shear Rebar Layout

You can choose to manually create the reinforcement layout for the column. This must be done if the user wishes to take advantage of bundled bars, multiple layers of reinforcement, or an unequal number of bars per face.  See the section on the Concrete Database and Rebar Layouts for more information.  If 'Default' is specified, then the program designs for an equal number of bars in each face of the rectangular column and may vary that reinforcing based on ACI minimums, maximums and the moment and shear demand at each section along the span.

Icr Factors (Cracked Moment of Inertia Factors)

The Icr Factor is used to reduce the bending stiffness of concrete columns per ACI 318-14 Table 6.6.3.1.1(a) (ACI 318-11 Section 10.10.4.1). If this entry is left blank, default values of 0.35 for beams and 0.70 for columns are used. 

Service Level Stiffness - Columns

Due to cracking and material non-linearity, modeling the stiffness of concrete members is more complex than it is for steel or wood members.

For typical applications, ACI 318-14 Section 6.6.3.1 (ACI 318-11 Section 10.10.4) requires that member stiffness be reduced to account for the cracking that occurs when a member is subjected to ultimate level loads. As described in the previous section, RISA uses the Icr Factor to account for this stiffness reduction. However, for service level analysis, the level of cracking is significantly less. Therefore, the stiffness used in your analysis should be representative of the reduced loading and reduced cracking. Per the ACI 318-14 Section R6.6.3.2.2 (ACI 318-11 Section R10.10.4.1), the program accounts for this increased stiffness by applying a factor of 1.43 to the cracked section properties for any load combination that has the “Service Load” flag checked on the Design tab of the Load Combinations Spreadsheet.

Concrete Design Parameters - Beams

The Concrete Beam tab on the Members Spreadsheet records the design parameters for the code checks of concrete beams. These parameters may also be assigned graphically. See Modifying Member Design Parameters to learn how to do this.

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The following parameters can be defined for each concrete member.

Label 

You can assign a unique Label to all of the members. Each label must be unique, so if you try to enter the same label more than once you receive an error message. You can Relabel options on the Tools menu.

Shape

The member Shape or Section Set is reported in the second column.  This value is listed for reference only and can not be edited as it is dictated by the entry in the Section/Shape column on the Primary tab.

Length

The member Length is reported in the third column.  This value can not be edited as it is dependent on the member end coordinates listed on the Primary Data tab. It is listed here as a reference only.

Flexural and Shear Rebar Layout 

You can choose to manually create the reinforcement layout for the beam. This must be done if the user wishes to take advantage of compression steel, or multiple layers of reinforcement. See Concrete Database - Rebar Layouts for more information. If Use Design Rule is specified, then the program designs for one layer of reinforcing and may vary that reinforcing based on ACI minimums, maximums, and the moment and shear demand at each section along the span using the Member Design Rules as the parameters of the reinforcement selection. If you define your own rebar layout, and compression reinforcement is defined, then the program considers the compression reinforcement in the analysis.

Icr Factors (Cracked Moment of Inertia Factors)

The Icr Factor is used to reduce the bending stiffness of concrete beams.

For ACI and Canadian codes (ACI 318-14 Section 6.6.3.1 and A23.3-04 section 9.2.1.2), if this entry is left blank, the Icr factor defaults to a value of 0.35 for beams and 0.70 for columns.

For Australian and New Zealand codes (per section 6.6.2 of AS3600-2001), this defaults to a value of 0.4 for beams and 0.8 for columns.

For Indian and Saudi codes, this entry defaults to a value of 1.0 for beams and columns.

Service Level Stiffness - Beams

Due to cracking and material non-linearity, modeling the stiffness of concrete members is more complex than it is for steel or wood members. 

For typical applications, ACI 318-14 Section 6.6.3.1 (ACI 318-11 Section 10.10.4) requires that member stiffness be reduced to account for the cracking that occurs when a member is subjected to ultimate level loads. As described in the previous section, RISA uses the Icr Factor to account for this stiffness reduction. However, for service level analysis, the level of cracking is significantly less. Therefore, the stiffness used in your analysis should be representative of the reduced loading and reduced cracking. Per ACI 318-14 Section R6.6.3.1.1 (ACI 318-11 Section R10.10.4.1), the program accounts for this increased stiffness by applying a factor of 1.43 to the cracked section properties for any load combination that has the “Service Load” flag checked on the Design tab of the Load Combinations Spreadsheet.

T-beam & L-beam Sections

T-beams and L-beams can be specified by assigning effective slab widths and slab thicknesses for the left and right side of the beam on the Concrete Beam tab of the Members Spreadsheet. These modifications can also be made graphically via the Modify Properties tab of the Draw Members tool.

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RISA-3D automatically trims the effective slab widths, B-eff Left and B-eff Right, to the maximum values indicated in ACI 318-14 Table 6.3.2.1 (ACI 318-11 Sections 8.12.2(a) and 8.12.3(a) & (b)) if the value entered by the user is greater than that allowed by the code. It should be noted that RISA-3D does not check ACI 318-14 Table 6.3.2.1 portions referring to adjacent framing (ACI 318-11 Sections 8.12.2(b) and 8.12.3(c)) because no adjacent framing checks are performed.

If the values of either B-eff Left or B-eff Right are left blank, a value of zero is assumed, indicating no additional slab width beyond 1/2 the beam width on that side.

Parabolic vs. Rectangular Stress Blocks 

You can specify whether you want your concrete design to be performed with a rectangular stress block, or with a more accurate parabolic stress block. While most hand calculations are performed using a rectangular stress block, the parabolic stress block is more accurate. In fact, most of the PCA design aids are based upon the parabolic stress distribution. A good reference on the parabolic stress block is the PCA Notes on ACI 318-99.

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Biaxial Bending of Columns 

You can specify whether you want your column design to be performed by using Exact Integration, or by using the PCA Load Contour Method. While most hand calculations are performed using the Load Contour Method, this method is merely an approximation based on the uniaxial failure conditions and the Parme Beta factor. In contrast, the Exact Integration method uses the true biaxial strain state to design the member. A good reference on the Load Contour Method is chapter 12 of the PCA Notes on ACI 318-99.

 

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British Eurocode Design Parameters (BS EN 1992-1-1: 2004)

General

• f’_ck – Can not be more than 50 MPa (7252 psi) for normal strength concrete
• α_cc is assumed to be 1 (recommended value) : See 3.1.6
• Effective length of T and L: L_o=.7*span length and b_eff,i =L_o/10
• φ_concrete = 1.5
• φ_rebar = 1.15
• Maximum bar spacing for beams = 300 mm

Tension Development Length

• α_ct= 1 (assumed in Eq 3.16)

• η₁= η₂=1 (assumed in Eq 8.2 to calculate bond stress)

• λ₃, λ₅, λ₄=1 (assumed in Eq 8.4)

• C_d in Table 8.2 assumed to be 1 bar diameter rebar spacing

• Development length when hooks are provided uses same assumptions as BS 8110-1: 1997

Shear Capacity of Concrete

To compute the shear capacity of concrete the following recommended values are being used:

• C_Rd,c =0.18/γ_c for Eq 6.2.a

• v_min =0.035 k^3/2 f_ck^1/2

• ν =.6*[1- f_ck /250]

• V_max is calculated from Eq. 6.5.

• θ is assumed 45 degrees in Eq 6.8.

Slender Column Design

• Biaxial column design done using Eq. 5.39

• Design based on nominal curvature

• λ_lim = 20 A B C/n^1/2 (A=.7, B= 1.1, C=0.7 for unbraced)

• K_ϕ =1 in Eq 5.34; the effect of creep is neglected.

Limitations 

Limitations - General

Torsion – Beams and columns ignore torsion with respect to the design of shear reinforcement. A message is shown in the detail report to remind you of this. You can turn the warning messages off on the Concrete tab of the Model Settings Dialog. However, when using the 2002 and newer ACI 318 code the program does check the torsion on the member against the Threshold Torsion value (see ACI 318-14 Section 22.7.4.1 and ACI 318-11 Section 11.5.1). A warning is produced, implying that the shear reinforcement has to be designed by the engineer for torsion.

Creep / Long Term Deflections – No considerations are taken in the analysis to account for the effects of creep or long term deflections.

Beam Design – Beams are not designed for weak axis y-y bending, weak axis shear, or axial forces. A message is shown in the detail report to remind you of this. You can turn the warning messages off on the Concrete tab of the Model Settings Dialog. Beams currently do not consider any compression steel in the calculation of the moment capacity. Beam "skin reinforcement" per the requirements of ACI 318-14 Section 9.7.2.3 (ACI 318-11 Section) 10.6.7 for beams with "d" greater than 36" is currently not specified by the program. The provisions in ACI 318-14 Section 9.9 (ACI 318-11 Section 10.7) for deep beams are not considered.

Column Design – Columns with biaxial moment and no axial load are currently designed using the PCA Load Contour Method even if Exact Integration is selected on the Model Settings dialog. This is shown on the detail report.

Limitations - ACI 

Shear Design –When ACI 318-19 is selected, the shear strength of concrete (Vc) uses equations in Table 22.5.5.1. Note that for members meeting the minimum shear reinforcement requirement (Av≥Av,min), Vc is taken as the larger of the results calculated by the equations (a) and (b) in the table. ACI 318-19 code suggests ρw can be taken as the sum of the areas of longitudinal bars located more than two-thirds of the overall member depth away from the extreme compression fiber. Therefore, RISA calculates ρw as the sum of the areas of longitudinal bars on the tension face.

When other ACI 318 editions are selected, the shear strength of the concrete alone is limited to the standard 2*λ*sqrt (f'c) equation from ACI 318-14 Section 22.5.5.1 (ACI 318-11 Section 11.2.1.1) and does not use the more detailed calculations of ACI 318-14 Table 22.5.5.1 (ACI 318-11 Section 11.2.2). Also, note that for members with significant axial tension (greater than 0) the program designs the shear reinforcement to carry the total shear per ACI 318-14 Section R22.5.7.1 (ACI 318-11 Section 11.2.1.3).

Deep Beam Design – The program does not design deep beams as defined in ACI 318-14 Section 9.9.1.1(a) (ACI 318-11 Section 10.7).

Threshold Torsion - The program does not adjust the threshold torsion value for the presence of axial force in a beam, though it does do this for columns.

Limitations - Canadian Code

Concrete Stress Profile –  Concrete stress strain curve (parabolic) is assumed same as PCA method for the Canadian codes.

Bi-Axial Bending - The program uses the simplified uniaxial solution provided in the Canadian specification rather than performing a complete biaxial condition.

Mid-Depth Flexural Strain for Shear Design - The program uses the code equation (per the General Method) to calculate exwith the moment and shear at the section taken from the envelope diagrams. The maximum ex for each span is conservatively assumed for the entire span. Currently the program has no option for pre-stressing, so V_p and A_p are both taken as zero.

Shear Design - The shear strength of concrete is calculated using β and θ, which are both calculated per the General Method (Clause 11.3.6.4 from the 2004 CSA A23.3). S_ze is calculated per equation 11-10 and a_g is always assumed to equal 20 mm (maximum aggregate size).

Limitations - Australian and New Zealand Codes 

Concrete Stress Profile –  Concrete stress strain curve (parabolic) is assumed same as ACI for the New Zealand and Australian codes.

Neutral Axis Parameter – Ku in AS code is always assumed to be less than 0.4.

Rebar Spacing –  NZS and AS codes: max spacing of rebar (beam) is 300 mm and minimum spacing is one bar diameter or 25mm whichever is bigger.

Shear Strength in Beams –  In AS code, when calculating the shear strength of a beam β2, β3 are always assumed to be unity.  This is always conservative for beams with little axial load, or beams in compression.  But, may be unconservative for members subjected to significant net tension.

Bi-Axial Bending – The New Zealand code does not appear to give a simplified method for solving biaxial column design.  Therefore, the PCA load contour method is being used instead.

Shear Tie Spacing –  Column/beam shear tie spacing is based on (a) and (c) of NZS 9.3.5.4 :1995.

Development Length – Development length in NZS is based on NZS 7.3.7.2 where αa is conservative assumed to be 1.3 (top bars) for all cases.  For the AS code, it is assumed that K1=1 and K2=2.4 in clause 13.1.2.1 of AS 3600:2001.

Slender Column Calculations –  EI is assumed to be equal to 0.25EcIg (with βd =0.6) in slender column calculations in AS and NZS codes (like in ACI).

Limitations - British

Concrete Stress Profile –  Concrete stress strain curve (parabolic) is taken from the British specification.  

Cracked Sections –   Icracked defaults to 1.0 for the British code. But, you can enter a value if you want. Service level stiffness is assumed to be 1.43 times the strength stiffness, but is not allowed to exceed Igross.

Bi-Axial Bending – The program uses the simplified uniaxial solution provided in the British specification rather than performing a complete biaxial condition.  

Limitations - Euro

Concrete Stress Profile –  Concrete stress strain curve (parabolic) is taken from the EuroCode specification.  

Cracked Sections –  Icracked defaults to 1.0 for EuroCode. But, you can enter a value is you want. Service level stiffness is assumed to be 1.43 times the strength stiffness, but is not allowed to exceed Igross.

Bi-Axial Bending – The program uses the simplified uniaxial solution provided in the EuroCode rather than performing a complete biaxial condition.

Limitations - Indian

Concrete Stress Profile –  Concrete stress strain curve (parabolic) is taken from the Indian specification.  

Cracked Sections –   Icracked defaults to 1.0 for the Indian code. But, you can enter a value if you want. Service level stiffness is assumed to be 1.43 times the strength stiffness, but is not allowed to exceed Igross.

Bi-Axial Bending – The program uses the simplified uniaxial solution provided in the Indian specification rather than performing a complete biaxial condition.  

Limitations - Saudi Code

Concrete Stress Profile –  Concrete stress strain curve (parabolic) is assumed to be the same as the ACI code.

Shear Strength–  The shear strength is based on 11.3.1.1 and does not include the more detailed provisions of section 11.3.1.2.

Yield Strength of Shear Ties - The yield strength of shear ties is not allowed to exceed 420MPa.

Shear Tie Spacing - Minimum spacing of shear ties is set to 50mm

Bi-Axial Bending – Both the Exact Integration and the PCA Load Contour methods for bi-axial bending are supported in the Saudi code.

Special Messages

In some instances code checks are not performed for a particular member. A message is usually shown in the Warning Log and Detail Report explaining why the code check was not done. There are also instances where a code check is performed, but the results may be suspect as a provision of the design code was violated. In these cases, results are provided so that they can be examined to find the cause of the problem. Following are the messages that may be seen.

No Load Combinations for Concrete Design have been run.

None of the load combinations that were run had the Concrete Design box checked on the Design tab of the Load Combinations Spreadsheet. Since there are no concrete design specific load combinations, there are no results or force diagrams to show.

Warning: No design for spans with less than 5 sections.

Certain very short spans in physical members can end up with less than 5 design sections. No design is attempted without at least 5 sections because maximum values can be missed and an un-conservative design may result.

Warning: No design for spans less than 1 ft.

Certain very short spans in physical members can end up with lengths less than 1 foot. No design is attempted for these sections.

Warning: Member is slender and can sway, but P-Delta Analysis was NOT run.

Under older ACI codes slender sway members need to be run with the P-Delta option turned on to account for secondary forces and moments. In some situations, a preliminary design without P-Delta is useful and so a design is performed and this warning is shown to remind you to run the final analysis including P-Delta effects. Alternately, if you’re using the redesign feature, the next suggested column may resolve this issue if it’s not slender.

Warning: Slender Compression Failure (Pu > .75Pc). No Slender calculations done.

Since RISA-3D allows you to specify a starting column size, it’s possible that for slender columns under substantial axial load you'll exceed the critical buckling load used in the slenderness equations in ACI 318-14 Section 6.6.4.5.2 (ACI 318-11 Section 10.10.6). Design results are still shown so the suggested shapes can be used to pick a new suggested column size that will not have this problem. Note that the design results shown are NOT valued because the slender moment effects have NOT been considered.

Warning: KL/r > 100 for this compression member.  See ACI 318-05 Section 10.11.5

Members that violate the KL/r limit still have design results calculated and shown. If you’re using the redesign feature, the next suggested shape should resolve this problem. Note this is only checked for the 2005 code and older.  Newer codes require a P-Delta analysis and omit this consideration.

Warning: Exact Integration selected but PCA method used

This message is shown when you've requested the Exact Integration option on the Model Settings Dialog, but we weren't able to converge a solution for the column in question. When Exact Integration does not converge, the PCA Method is used instead to give an idea of the demand vs. the capacity.

Warning: PCA Method Failed. Axial Load > Axial Capacity.

One of the limitations of the PCA Method is that it requires the column being checked to have a greater axial capacity than the axial demand. Since RISA-3D allows you to set a starting size, it’s possible that the demand may be greater than the capacity. In this case a very rough estimate of the capacity is calculated by using the independent moment capacity about each axis considering the axial load. The resulting code check value is then based on the combined demand vector over the combined capacity vector and is always greater than 1.0. The purpose of the results in this case is to show the column failed, not to give an accurate estimate of the over-demand. The redesign feature suggests a larger shape to resolve this issue.

Warning: The shear tie spacing does not meet the code Minimum Requirement

This warning is stating that either minimum spacing or strength requirements are not being met for the shear reinforcement in the concrete member.

P-Delta analysis required for all ACI 318-14/11/08 Load Combinations

A second order analysis is required as of the 2014, 2011 and 2008 editions of the ACI 318 code. A code check is only be given if P-Delta is turned on in the Load Combinations spreadsheet, or if this requirement is intentionally waived in the Application Settings.